With the unlimited increase in the integration of semiconductor integrated circuits, semiconductor process techniques for the manufacture of such circuits have become increasingly finer, and have burst into the quarter-micron age from the sub-half-micron age. As a result, the numerical aperture (NA) of exposure apparatuses used in photolithographic exposure processes has increased, and as a result, the focal depth of such exposure apparatuses has become increasingly shallow. Furthermore, there has also been an increased tendency toward three-dimensionalization of device structures, the formation of multi-layer structures in electrode wiring, and increased complexity of such structures.
In recent years, CMP (chemical-mechanical polishing or chemical-mechanical planarization), which is a global flattening technique for inter-layer insulating films in semiconductor processes, has attracted attention as an important technique for dealing with such trends. In the substrate polishing apparatus used in this CMP process, as is indicated by 1 in FIG. 11, a substrate (semiconductor wafer) 107 which is mounted in a substrate holding part 102 is caused to undergo relative motion while being pressed against a polishing pad 103 that is fastened to a polishing surface plate 104, and the surface of the substrate is globally polished by the chemical polishing action and mechanical polishing action of a polishing agent (slurry) 105 that is supplied from a polishing agent supply mechanism 106.
Measurement of the residual film thickness in polishing and ascertainment of the endpoint of the process are among the most important performance requirements in such a substrate polishing apparatus. The precision of this measurement greatly influences the quality of the semiconductor elements that are manufactured by means of this apparatus, and therefore the quality of the integrated circuits.
However, conventional substrate polishing apparatuses are all on an extension of existing apparatuses, and cannot currently satisfy requirements for an increased degree of working precision. Especially in regard to variation in the residual film thickness between lots, fluctuating factors in the amount of polishing per unit time (polishing rate), e.g., various factors that fluctuate on an occasional basis such as the polishing pressure, amount of polishing agent supplied and temperature environment of the substrate, etc., in addition to clogging of polishing pad, cannot be handled in the case of control methods that depend on the setting of the working time.
Furthermore, methods have also been used in which the residual film thickness following working is measured by means of a special measuring device (ellipsometer, etc.), and the residual film thickness is controlled by feeding this information back into the substrate polishing apparatus. However, this method suffers from a drawback in that the polishing work must be temporarily stopped in order to perform measurements. Furthermore, even if an accurate residual film thickness value is obtained for the polished substrate by such measurements, the measurements are performed intermittently; as a result, under conditions in which the above-mentioned fluctuating factors are present, accurate ascertainment of the endpoint of the process is impossible. Accordingly, the object of accurately obtaining the target residual film thickness cannot be achieved, and variation in the film thickness between lots cannot be ignored.
In the past, therefore, besides methods in which the polishing endpoint is detected by time control, detection methods that utilize fluctuations in the torque of the motor that drives the polishing surface plate have been proposed as detection methods for detecting the polishing endpoint at the same time that polishing is performed (i.e., in-situ). Such methods utilize the fact that the polishing resistance varies when the material of the polished surface of the substrate varies at the polishing endpoint. Fluctuations in the polishing resistance are detected by monitoring the motor torque, and the polishing endpoint is detected from variations in the motor torque.
However, although detection methods that utilize fluctuations in the motor torque are effective in cases where a variation in the material occurs at the polishing endpoint (e.g., in cases where the underlying silicon is exposed in the process of polishing an oxide film), such methods are insufficient in terms of precision in cases where it is desired to flatten irregularities in the surface of the same film consisting of the same material throughout with high precision (approximately ±100 nm or better). Furthermore, since no conspicuous fluctuation in the motor torque appears at the polishing endpoint in such cases, detection of the polishing endpoint is virtually impossible.
Recently, therefore, the development of endpoint detection based on an optical system has been pursued instead of endpoint detection from such torque fluctuations.
A powerful example of such an optical endpoint detection technique is shown in FIG. 12. In FIG. 12, the polishing apparatus 1 produces a relative motion by the rotary motion 100 of the substrate and the rotary motion 101 of the polishing pad while pressing the substrate (semiconductor wafer) 107 which is mounted on a substrate holding part 102 against the polishing pad 103, which is fastened to a polishing surface plate 104, and the surface of the substrate is globally polished by the chemical polishing action and mechanical polishing action of a polishing agent (slurry) 105 that is supplied from a polishing agent supply mechanism 106.
In this technique, probe light emitted from a monitoring apparatus 109 is caused to illuminate the semiconductor wafer 107 via transparent windows 110 formed in the polishing pad 103 and polishing surface plate 104, and ascertainment of the process endpoint is accomplished by the reception of reflected light from the semiconductor wafer 107 by a photo-detection device with which the monitoring device is equipped.
However, in regard to such an endpoint detection method, only the scope of the method in terms of the principle of the method has been disclosed; there has been no clear disclosure regarding the disposition of constituent members such as the concrete optical system. For example, the technique described in Japanese Patent Application Kokai No. H9-36072 may be cited as an example of a technique that is close to the technique shown in FIG. 12; however, there is no description of the construction of the optical sensor in this patent.
Furthermore, as is seen from FIG. 12, the monitoring device 109 must be fastened to the rotating polishing surface plate 104. Since the monitoring device is equipped with a light source and a photo-detector, an accommodating space of a size that cannot be ignored is required in the lower part of the polishing surface plate 104 in order to accommodate this monitoring device in cases where the monitoring device rotates. This greatly restricts the design of the CMP polishing apparatus.
Generally, in the case of equipment such as a CMP polishing apparatus that is operated inside an expensive clean room, there is an especially strong demand for reduction in the size and weight of the apparatus. Thus, such an increase in the accommodating space not only reduces the degree of freedom in design, but is also a major obstacle to size and weight reduction of the CMP polishing apparatus.